The present invention relates in general to electro-optical document reading systems such as facsimile equipment. More particularly, the present invention relates to a mask for controlling image plane light level distribution in line scan optical imaging systems. Still more particularly, the present invention relates to a linear strip mask for controlling the illumination of a line of text to be read by a line scan optical imaging system.
In many electro-optical document reading devices, light is directed onto a line of a printed document, and light is reflected from each elemental area of the line of the printed document in accordance with the color or blackness of the elemental area. The reflected light is fed through an optical system, usually including a spherical lens system, to electrical apparatus in which the reflected light is converted to electrical signals which are used to reproduce the document at a remote location. Typical of such an electrical apparatus is a photoelectric sensor, positioned behind the lens, which converts light reflected from each unit area of a line of print on the document to electrical signals. One suitable photoelectric device comprises an integrated circuit chip having a large number of tiny photosensitive elements arranged in a line, each element receiving light from a unit area of each line of print.
In the prior art, various light sources have been utilized to illuminate the line of text being scanned. Thus, in U.S. Pat. No. 4,220,978, an electro-optical document reader similar to that previously discussed teaches that either incandescent lamps, fluorescent lamps or light-emitting diodes may be utilized as a source of illumination.
In all of the prior art systems, means must be included to provide a substantially uniform light distribution at the image plane of the electrical apparatus which converts the reflected light to electrical signals. To provide this uniform distribution at the image plane of the electrical apparatus, a light distribution at the object plane (or document) that is brighter at the edges must be provided in order to compensate for the cos.sup.4 drop-off of the lens system. Further, in a system employing a fluorescent tube to illuminate a line of text, the light intensity output along the length of the tube is roughly of a sinusoidal pattern. That is, more light is output at the center of the tube's length than at its ends. Thus, if a line of printed text were positioned parallel to the fluorescent tube, more light would hit the words at the center of the line than the words at either end of the line. As a consequence, assuming the black/white content was identical along the entire line of text, more light will be reflected off points at the center of the line of text than from points at the ends of the line of text. Since the electrical apparatus detects black and white areas along the scan line based on the amount of reflected light, the use of an uncorrected fluorescent tube output to illuminate a scan line would require that the light sensitivity of each element in the electrical apparatus be tailored to its position with respect to the line of text. Since such customization would be quite expensive to implement, various techniques have been proposed to provide a substantially uniform light distribution at the input to the electrical conversion apparatus.
In one embodiment of the system taught in U.S. Pat. No. 4,220,978, two incandescent lamps are disposed close to the rear opening of a light guide which directs light toward the document. The lamps are spaced apart so that some of the light rays from the lamps strike the document directly and some strike the document through reflections from the side walls and the top and bottom plates of the light guide, so that the total light which reaches the document has the desired distribution of light intensity across the document; viz., it is brighter at the document edges. In this embodiment, light reflected from the document is fed back through the light guide, then through a lens and onto a photoelectric sensor. The disadvantages of this system is in the fact that a light guide is required, increasing the system's cost. In addition, the system is relatively large due to the required length of the light guide.
In order to do away with the need for a light guide, various systems utilizing a fluorescent tube have been suggested. In all known prior art systems utilizing a fluorescent light source, the illumination subsystem and imaging subsystem are not in the same plane as the most direct path from the light source to the document. Therefore, there are two light paths in these systems which may be modified in order to obtain a uniform light distribution at the image plane. Thus, a first option is to modify the light output from the fluorescent tube before it reaches the document. The second option is to modify the reflected light pattern between the document and lens or electrical detection apparatus.
In U.S. Pat. No. 4,220,978, a system is disclosed wherein a single fluorescent bulb is suitably masked by an opaque coating to allow greater light output at its ends than at its center. By using the opaque coating, the light intensity at a distance from the bulb has the appropriate distribution on the scan line of the document. In this system, the bulb is positioned next to a light guide which directs the light to the document. Light reflected from the document does not return through the light guide, but travels along a path disposed outside the light guide. Thus, the light guide is disposed at such an angle to the document that light is reflected from the document on an axis which is disposed above the light guide to a suitable optical apparatus and electro-optical pickup mechanism. Although this system solves the problem of providing the appropriate light intensity along the scan line, it requires that the fluorescent tubes be suitably coated with the opaque mask. In addition, this system requires the use of a light guide which enlarges the size of the system and adds to its cost.
Systems utilizing a fluorescent tube, but not requiring a light guide, are also known in the prior art. In such systems, the fluorescent tube is positioned parallel to and above the line of text to be scanned, and in close proximity to the document. The light from the tube thus travels down at an acute angle to illuminate the scan line. A lens is positioned a distance from the document at the same height as the scan line. The lens focuses the reflected light onto an array of CCD sensors positioned behind the lens. In this system, the nonlinear light intensity output of the fluorescent tube is compensated for by one of two methods.
In the first method, a line aperture is used which has an opening which varies in size across the length of the aperture. Thus, the aperture opening is narrow at the center and widens as one moves towards the ends of the aperture. This aperture is positioned in the reflected light path between the scan line on tne document and the lens. This variable size aperture thus controls the amount of light entering the lens from each point in the object plane (viz., the document scan line).
The first method suffers from two major drawbacks. The aperture must be accurately positioned along the axis normal to the scan line axis. If not positioned correctly, the light level will not be as desired. The second drawback is the reduction of modulation transfer of the imaging system due to the effect of an obscured lens aperture.
The second method utilizes a photographically produced continuous tone gray-scale gradient filter which varies in density across the length of the filter. As in the first method, the line filter is positioned between the lens and the object plane where it controls the amount of light entering the lens from each point on the object plane.
The second method also suffers from two major drawbacks. The varying density filter creates a reduction in the modulation transfer of the imaging system due to the scattering of light by the photographic emulsion. The second major drawback is the excessive cost of continuous tone gray-scale gradient filters.
It is the general object of the present invention to overcome the above-mentioned drawbacks of the prior art by providing an improved apparatus for controlling the image plane light level distribution in line scan optical imaging systems.
It is another object of the present invention to provide a repeatable and low cost apparatus for controlling the light level distribution in line scan optical imaging systems.
It is yet another object of the present invention to provide a mask for modifying the light intensity output from a light source.
It is a further object of the present invention to provide a fluorescent tube mask which compensates for the non-uniform light output along the length of a fluorescent tube and which need not be accurately aligned in the axis normal to the scan line axis.
It is still another object of the present invention to provide a mask for controlling light level distribution in line scan optical imaging systems which does not reduce imaging system modulation transfer.
It is still a further object of the present invention to provide a low cost and compact system for controlling light level distribution in line scan imaging systems.
It is yet another object of the present invention to provide a fluorescent tube mask which compensates for the cos.sup.4 drop-off of reflected light prior to transmission through the lens of a line scan optical imaging system.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment when read in conjunction with the drawings.